Stem of Angiopteris evecta . 513 
steles (Figs. 23 and 25) the protoxylem is found in groups of 
two or more spiral tracheides situated along the periphery 
and in the centre of the stele; the number of protoxylem- 
groups depends upon the size of the stele, the larger steles 
containing about five or six such groups. 
The earliest protoxylem appears along the periphery of 
the stele ; that is, no steles occur in which the central groups 
are unaccompanied by some at the periphery. The proto¬ 
phloem arises on the outer side of the stele in the form of 
a discontinuous arc of small, somewhat thick-walled elements 
(Fig. 23 pp) which, from the presence of minute granules on 
their walls, and from their method of attachment to each 
other, appear to be small sieve-tubes. The position of the 
protophloem is shown diagrammatically in Fig. 2, and is 
present in Fig. 25 as a somewhat indistinct layer interior to 
the large sieve-tubes on the left side of the section. 
In a young stele the protophloem and protoxylem are seen, 
roughly speaking, at the opposite ends of the same radius, 
the protophloem being towards the periphery of the stem. 
In very young steles, however, the arc of phloem is found 
differentiated before any protoxylem elements have been 
lignified, and this is particularly striking in the steles 
in the young leaf, where the sieve-tubes are of considerable 
size. 
The arc of protophloem is never completed round the stele, 
but the next stage in the development of the tissues after 
the appearance of the protoxylem is the differentiation of 
large sieve-tubes exterior to the protophloem. A continuous 
ring of sieve-tubes is finally formed round the xylem, but in 
certain stages of development, for example in that represented 
in Fig. 23, the sieve-tubes exterior to the protophloem are 
larger and thicker-walled than those in the neighbourhood of 
the peripheral protoxylem. 
The lignification of additional tracheides takes place at the 
same time as the formation of the ring of large sieve-tubes, 
and in the mature stele we find the concentric structure of the 
ordinary Fern-type. The ring of sieve-tubes is more con- 
M m 
